outline the main stages in the process of cell signalling leading to specific responses: secretion of specific chemicals (ligands) from cells, transport of ligands to target cells, binding of ligands to cell surface receptors on target cells

Fluid Mosaic Membrane – Overview

The plasma membrane is described by the fluid‑mosaic model. It consists of a phospholipid bilayer in which proteins, cholesterol and carbohydrates are embedded or attached, giving the membrane both fluidity and structural diversity. This architecture allows receptors to move laterally, cluster, and interact with extracellular signalling molecules (ligands).

Cell Signalling – Main Stages Leading to Specific Responses

1. Secretion of Specific Chemicals (Ligands) from Signalling Cells

• Ligands are often hormones, neurotransmitters, cytokines or growth factors.
• Production occurs in the signalling (donor) cell’s cytoplasm or endoplasmic reticulum.
• Packaging into secretory vesicles involves:

  1. Sorting in the Golgi apparatus.
  2. Vesicle budding and transport along microtubules.

• Exocytosis releases the ligand into the extracellular fluid (ECF).

2. Transport of Ligands to Target Cells

• Ligands travel through the ECF or bloodstream, depending on the organism.
• Transport mechanisms include:

  • Simple diffusion for small, lipophilic ligands.
  • Carrier proteins (e.g., albumin) for hydrophobic hormones.
  • Active transport or vesicular transport for larger peptide ligands.

• Degradation enzymes in the plasma can terminate the signal by breaking down the ligand.

3. Binding of Ligands to Cell‑Surface Receptors on Target Cells

• Receptors are integral membrane proteins that span the phospholipid bilayer.
• Binding sites are often located in extracellular domains that protrude from the membrane.
• Ligand‑receptor interaction triggers conformational changes that:

  • Activate intracellular signalling cascades (e.g., G‑protein activation, receptor tyrosine kinase autophosphorylation).
  • Result in specific cellular responses such as gene expression, enzyme activation, or ion channel opening.

• Receptor internalisation (endocytosis) can modulate signal strength and duration.

Integrated Sequence of Events

1. Signal synthesis → packaging → vesicle formation.
2. Vesicle transport to plasma membrane → exocytosis → ligand release.
3. Ligand diffusion/transport through extracellular space.
4. Ligand encounters target cell → binds to specific surface receptor.
5. Receptor activation → intracellular second‑messenger cascade → cellular response.
6. Signal termination by ligand degradation, receptor desensitisation, or phosphatase activity.

Summary Table of the Three Main Stages